Amplifying system provided with parallel electron discharge paths having different mutual conductances



1950 G. H. P. ALMA ETAL I AMPLIFYING SYSTEM PROVIDED WITH PARALLEL ELECTRON DISCHARGE PATHS HAVING DIFFERENT MUTUAL CONDUCTANCES Flled July 15, 1946 I N VEN TORS. efrEEMTHH/DRMPIYZRUSALM PIE THE WEEK M/VKZDZK l atented Nov. 21, 1.950

PARALLEL ELECTRON DISCHARGE PATHS HAVING- DIFFERENT MUTUAL CON DUCTAN CES Gerrit Hendrik PetrusAlmaand Pieter Frederik; van Eldik, Ein'dlioven, Netherlands, assignors to Hartford'N ation'al Bank and TfustComp'a'ny, Hartford; (301111., astrustee Application July 15, 1946, Serial No. 683,710 In the Netherlands March 15, 1943 Section 1, Public'Law 690; August 8', I946 Patent'expires March 15, 1963 8 Claims.

The invention relates" to a circuit-arrangement for amplifying electrical oscillations by means oi atleasttwo discharge pathssconnected in parallel and controlled by control grids, .said discharge paths havingcharacteristics whose slopes asfunctions of the control-gridv voltages have different courses.

An amplifying circuit-arrangement generally has to meet the requirement that it should: be possible to control the. amplification over a large range'withoutvthe productionof distortion, Besides, the) circuit-arrangement frequently has to meet the requirementthat in the uncontrolled condition, i.. e. at comparatively low values of the oscillations to be a1nplified,iit should yield a very large amplification.

In practice it is impossible simultaneously to meet these two requirements, for the discharge tubes utilized in the amplifying circuitarrangements exhibit either a characteristic which .has a great slope and a small grid con-* trol range or a characteristic which has a slight" slope .anda large grid control range. With tubes of the first type it is possible to obtain a largeamplification but the possibilities of control are very limited; since the lower portion of the characteristic concerned is excessively curved, which gives rise to an appreciable distortion. With tubes of the second type a satisfactory control is possible without the production of distortion, but the maximum amplification which can be obtained is only small. With these considerations it should beborne in mind that a comparison of the mutual conductances of different discharge tubes has only sense if the anode voltage is assumed to be given or which comes to the same thing, if a maximum anode current is fixed. In. the case of low-frequency amplification a high value of the mutual conductance has only sense if it is attained at a low value ofthe anode current since the value of the anodecurrent determines the amount of energy which must be destroyed in the resistance. In the case of high-frequency amplification it is also necessary that a high value of the "mutual conductance should be attended with a low value of theanode current since herethe value of the anode current'determines' the value of" the' anodenoise current.

The present invention provides acircuit ar 2, rangementwvhich not onl-y yieldsa large maxi-- mum amplification butalso has a large rangeof control;

According to the invention, use is made of' at least two' discharge paths which are con-' nected in parallel and which are controlled by control gridswhilst' the one discharge path has'a comparatively large average-mutual conductance and a comparatively small grid control rangewhereas the other path has a comparatively small average mutual conductance and a comparatively large grid control" range. Means are provided whichautomatically cause that at small amplitudes of theoscillations to be' amplified the amplification iseif'ected exclusivelyon almost exclusively by the first-mentionedpathand that" atlarge amplitudes of these oscillations the amplification is effected exclusively or almost exclusively by the last mentioned path.

Theautomatic change-over from the one disch'argepath to the other may efficiently be obtained by supplying to the: control grids a control voltage which depends upon the amplitude oftheoscillations to be amplified;- this control voltage acts atthesame time ori the potentialof an auxiliary electrode located in the lastmentioned discharge path (i. e. the path with the comparatively small average mutual conductance and the comparatively large grid-control range) in such manner that at comparatively low values of the oscillations to be amplified this path is blocked.

At high values of the'control Voltage the discharge path with the comparatively large average mutual cond'ucta-nce'and the comparatively small g-ridcontrol range: is blocked by the said control voltage itself. I

The said auxiliary elect'rodeispreferably connected to theinterconnected anodes of the two discharge-paths through'the' intermediary of a resistance whilst inthe' common anode circuit of the discharge paths is" incorporated a'r'esisi'ianc'e; preferably ofhigh value; which is so dimensioned that at small amplitudes of the oscil lation's to be amplified the discharge path in. which the auxiliary. electrode is located is" blocked;

which the auxiliary electrode is connected to' the anodes; is preferably so dimensioned" that" the resulting characteristic of the discharge paths exhibit a fluent course.

It is advisable to give the resistance through the intermediary of which the auxiliary electrode is connected to the anodes, a high value with respect to the resistance included in the anode circuit.

In some cases it is desirable to connect the auxiliary electrode, in addition, via a resistance, preferably of high value, to the interconnected cathodes of the two discharge paths.

The discharge paths are preferably provided in a single tube.

In one particularly suitable embodiment of the invention the discharge path in which the said auxiliary electrode is located is controlled by a control grid whose pitch of winding and/ or whose distance from the cathode are not constant. The other discharge path is preferably controlled by means of a control grid whose pitch of winding as well as whose distance from the cathode are constant.

Both discharge paths may be provided, for example, in one control pentode by equipping the latter with a screen grid which consists of two portions which are electrically insulated from one another, that portion which is located opposite the control-grid portion whose pitch of winding and/or whose distance from the cathode are not constant, forming the above-mentioned auxiliary electrode.

The circuit-arrangement according to the invention, is particularly suited for low-irequency amplification but may also be utilized for the amplification of high-frequency oscillations.

The invention will be explained more fully with reference to the accompanying drawing.

Fig. 1 represents a circuit-arrangement for amplifying low-irequency oscillations, wherein use is made of two discharge paths which are both provided in a discharge tube I. This tube comprises a control grid 2, a screen grid consisting of two portions and a suppressor grid 4, an anode and a ca.hode G which is connected to the suppressor grid i. The abovementioned auxiliary electrode is formed by the screen-grid portion 3. The discharge path in which this auxiliary electrode is located is controlled by a portion of the control grid which has been wound with a non-uniform pitch, the control grid portion located in the other discharge path being wound with a constant pitch. The two control grid portions are electrically connected to one another. The suppressor grid 5, the anode 5 and the cathode serve simultaneously for the two discharge paths.

Owing to the particular construction of the control grid, the left-hand discharge path (in which the auxiliary electrode 3' is located) exhibits a characteristic which has a comparatively slight average slope and a comparatively large grid control range whereas the right-hand discharge path exhibits a characteristic which has a comparatively great average slope and a comparatively small grid control range. Such a difference in the characteristics may also be brought about in another manner, for example by giving the auxiliary electrode 3 a particular construction.

The screen grid portion 3 is connected, through the intermediary of a potentiometer consisting of two resistances i and 8, to the positive terminal of a source of direct-current voltage (not shown in the drawing) and, through the intermediary of a condenser 8, to the cathode 6. The auxiliary cathode 3 is connected via a high resistance It to the anode 5 and via a condenser E I to the oathode 6. Furthermore, the anode circuit comprises a comparatively high resistance ii. The value or the resistance in exceeds as a rule the value of the resistance l2 and preferably it is even large with respect to the said value.

The oscillations to be amplified are supplied, jointly with a control voltage which depends upon the amplitude of these oscillations, to the control grid 2; the amplified oscillations may be taken through a condenser i3 from the resistance [2.

The working of the circuit-arrangement may be explained with reference to Fig. 2. This figure represents three characteristics which indicate the relation which exists between the anode current (ia) and the control grid direct-current voltage (V91) in three circuit-arrangements which slightly differ from one another.

0. Relates to the circuit-arrangement which is obtained by switching the left-hand discharge path out of circuit, which is achieved by interrupting the connection between the auxiliary electrode 3 and the anode 5 and, instead thereof, by connecting this electrode directly to the cathode 6.

b. Relates to the circuit-arrangement represented in Fig. 1.

c. Relates to the circuit arrangement which is obtained by connecting the auxiliary electrode 3 to the screen grid 3 and by interrupting the connection between the auxiliary electrode 3 and the anode 5.

The value of the anode current has been plotted along the vertical axis in ,u. milliamperes, the value of the control grid direct-current voltage being plotted along the horizontal axis in volts.

With the circuit arrangement mentioned under 0., it is only the right-hand discharge path that functions and the characteristic a is consequently the ordinary image of a pentode characteristic: a steep slope and a small grid control range; at the point P the path is already completely blocked. A discharge path with such a characteristic possesses, it is true, satisfactory properties as an amplifier, but the amplification can only be controlled to a very limited degree. In the case of amplification in the neighbourhood of the point P there occurs an inadmissibly high distortion of the oscillations to be amplified.

The circuit-arrangement indicated by 0 substantially corresponds to an amplifying circuitarrangement wherein use is made of a control pentode. This characteristic exhibits a gradual course of the slope and a very large grid control range; the maximum slope-at a given anode voltage or anode current-and therefore the maximum amplification that can be obtained are, however, comparatively small.

As appears from the foregoing, it has hitherto been impossible to realize a circuit-arrangement with a high maximum amplification wherein, moreover, the amplification is controllable over a large range without distortion.

These two requirements are satisfied, however, by the circuit-arrangement according to the invention. In this circuit-arrangement the right-hand discharge path acts in almost the same manner as in the circuit-arrangement mentioned under a. When this discharge path starts functioning (at the point P) the action of the left-hand discharge path steadily decreases automatically and almost immediately thereupon this path is completelyblocked. This blocking is effectedlin the .following; manner. Atva small am;-

pli-tude of the: oscillations to be amplified, he. at. a: low control voltage, the anode-.currentsteadtowhich the anode direct-current voltage and the voltage of the auxiliary electrode 3' greatly decrease. The voltage of theauxiliary electrode decreases in consequence thereof below the minimum value which is necessary for maintaining the discharge current, and the discharge path is blocked. The location of the point at which the blocking sets in, is chiefly determined by thevalue of the resistance l2.

At a large amplitude of the oscillations to be amplified, i. e. at a high value of the control voltage, the right-hand discharge path is in a condition. which is characterized in the characteristic bya point locatedv to the left ofv the point P-,, which consequently implies thattin" this case.- the right hand discharge path is blocked. According to the value of the. control voltage, automatical changing-over from the one discharge path to the other thus consequently obtained.

The operation of the circuit-arrangement according to the invention, is represented cy the characteristic b. This characteristic exhibits indeed not only quite satisfactory properties of control but it also yields a high maximum amplification at a low value of the anode current.

In the most curved portion of the curve the two discharge paths function simultaneously. By a proper choice of the resistances utilized, and more particularly of the resistance It, and by suitably dimensioning the discharge paths, it is possible to give the characteristic a fluent course so that any distortion that may occur remains at any rate within admissible limits.

With the circuit-arrangement according to the invention it is possible to obtain, for example, a 500-fold amplification whilst the amplification can be controlled in a ratio of l to 1000 (that is to say that at very large amplitudes the incoming oscillations are attenuated in a ratio of 1 to 2). In this case the distortion remains less than 1%, whilst the maximum anode current is less than lmA.

Fig. 3 represents another embodiment of the invention, which differs from the embodiment shown in Fig. 1 in that the auxiliary electrode 3' is connected to the cathode through a resistance l4 and that the voltage of the screen grid is directly taken through a resistance 8 from the source of anode direct-current voltage. With this circuit-arrangement it is possible to obtain an almost exponential control of the amplification. Such a control is particularly important for microphone amplifiers, with which it is desired to obtain a linear relation between the value of the control voltage and the value of the amplification expressed in (13.

By causing the value of the resistance M to vary, it is possible to modify the slope of the characteristic which indicates the relation between the logarithm of the slope and the control grid voltage.

If high-frequency oscillations are to be amplified, it is preferred to connect a circuit tuned to these oscillations in series with the resistance 12, this resistance itself being shunted by means of a condenser for the high-frequency oscillations.

What we claim is:

1. Apparatus for amplifying electrical oscillations comprising first and second electron disdetermined low amplitude level of said oscilcharge paths enclosed inan evacuatedenvelope arranged-in parallel relation, each of said paths" including, a. cathode a: control grid and an outputelectrode, said second path: further includ-' ing; an auxiliary: electrode, said first path having a relatively large average mutual conductance characteristicand a relatively small grid-- controlrange, said second path having a .relativelysmall mutual conductance characteristic and a relatively larger grid control rangeanoutput circuit including. a first resistance connect ed to the: outputelectrodes of said paths, means to apply the oscillationsto be amplified to the control grids: in' said' paths; a second resistance connectedzbetween saidoutput electrodes and the auxiliary'electrode inz'said second path, said first resistance having a. value: atiwhich the voltage. developed-L thereacrossand applied to said auxiliary electrodethroughsaid se'con'd resistance renderssaid second path non-conductive at apre- 2. An arrangement as set forth in claim 1, wherein said second resistance has a value at which the combined mutual conductance characteristics of said paths results in a curve free from discontinuity.

3. An arrangement as set forth in claim 1, wherein the value of said second resistance is large relative to the value of said first resistance.

4. An arrangement, as set forth in claim 1, further including a third resistance connected between said auxiliary grid and the cathodes of said paths.

5. Apparatus for amplifying electrical oscillations comprising an electron discharge tube including a cathode, a control grid having first and second portions possessing disparate characteristics, an anode and an auxiliary electrode interposed between said second portion of said grid and said anode; said cathode, said first portion of said grid and said anode defining a first discharge path having a relatively large average mutual conductance characteristic and a relatively small grid control voltage range; said cathode, said second portion of said grid, said auxiliary electrode and said anode defining a sec ond discharge path having a relatively small average mutual conductance characteristic and a relatively large grid control voltage range; an output resistance connected to the anode of said tube; an auxiliary resistance connected between said anode and said auxiliary electrode; and means to apply the oscillations to be amplified to said control grid whereby a control Voltage is applied to said auxiliary grid in accordance with the amplitude level of said oscillations, said output resistance having a value at which the voltage developed thereacross and applied to said auxiliary grid through said auxiliary resistance renders said second path non-conductive at a predetermined low amplitude level of said oscillations.

6. An arrangement, as set forth in claim 5, wherein said control grid is constituted by a coilshaped electrode, the first portion of which has a given pitch and the second portion of which has a different pitch.

7. An arrangement, as set forth in claim 5, wherein said control grid is constituted by an electrode, the first portion of which is closed to said cathode than the second portion thereof.

8. Apparatus for amplifying electrical oscillations comprising an electron discharge tube including a cathode, a control grid having first and second portions possessingdistinct characteristics, a screen grid having first and secondsaid suppressor grid and said anode defining a.

second discharge path having a relatively small average mutual conductance characteristic and a relatively large grid control voltage range; a first resistance; means to apply a positive voltage to said anode through said first resistance; a.

second resistance connected between said first section of said screen grid and the end of said first resistance which is remote from said anode; a third resistance connected between the second section of said screen grid and slid a dode; and means to supply the oscillations to be amplified to said control grid, said first resistance having a value at which the voltage developed thereacross and applied to said second section of said screen grid through said third resistance renders said second path non-conductive at a predetermined 10w amplitude level of oscillations.

' GERRIT HENDRIK PETRUS ALMA.

PIETER FREDERIK VAN ELDIK.

REFERENCES CITED lhe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,066,284- Ballantine Dec. 29, 1936 v 2,245,616 Soller June 17, 1941 2,263,825 Loughren Nov. 25, 1941 2,315,043 Boucke Mar. 30, 1943 

